Cutting tip for metal-removing processing

ABSTRACT

A sintered cutting tip composed of 
     70-90 weight percent of aluminum oxide; 
     10-30 weight percent of zirconium oxide; 
     0.1-0.5 weight percent of magnesium oxide, 
     and less than 0.6 weight percent of impurities, having a porosity of less than 2%, 
     an average particle size of less than 1.7 μm, and a fracture toughness K Ic  at room temperature of at least 190 N/mm 3  / 2  and of at least 140 N/mm 3  / 2  at 1000° C. 
     A method for manufacturing the cutting tip and its use are disclosed. Cutting tips in accordance with the present invention possess superior performance in terms of longer cutting life and are particularly suitable for use with case-hardened, quenched and tempered steels at high cutting speeds.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a cutting tip for removal of metal during theprocessing of metal parts, such as, shafts, axles, etc. Particularly, itpertains to cutting tips for case-hardened and quenched and temperedsteel, preferably for use at cutting speeds greater than 500 m/min,consisting of aluminum oxide with additions of zirconium oxide andsintered at a high temperature.

2. Description of the Prior Art

In recent decades, cutting tips based on aluminum oxide have proventhemselves in an outstanding manner for the machining of metals. Thus,of the cutting materials available, these cutting tips have foundsteadily increasing use especially for machining operations at highcutting speeds which result in high temperatures. Aluminum oxidepossesses a certain brittleness which decreases the life of the cuttingedges of the cutting tips. Thus, such tips are not entirely satisfactoryand in spite of the great hardness and wear resistance of the aluminumoxide cutting tips, efforts are still being made to further improve thismaterial.

Quite a number of proposals have therefore already been made for makingthe aluminum oxide based, relatively brittle cutting tips more ductileby means of additives, i.e., in order to increase their ultimatebreaking strength. Such additives may consist of metals which lead tothe so-called "cermets". Additions of metal carbides, nitrides andborides, such as, for example, titanium carbide, which particularlyincrease the wear resistance, have proven to be valuable.

An older proposal disclosed in German Auslegeschrift No. 23 07 654suggests using zirconium oxide as a material for cutting tips. Thiszirconium oxide is partially stabilized and has a cubic phase content of75-95%. In German Offenlegungsschrift No. 27 41 295, this suggestion istaken up once again with the variation that 0.5 to 35 weight percent ofthe partially stabilized zirconium oxide is intercalated in a matrix ofα-aluminum oxide. This provides an increase in the shape-stability ofthe cutting tips. Due to the relatively high stabilizer content, i.e., 4weight percent of calcium oxide, the heat resistance of these cuttingtips is decreased to such an extent that their performance at highcutting speeds is less than even that of a pure aluminum oxide ceramic.

The problems associated with these aluminum oxide cutting ceramicscontaining additives generally arise from the fact that the desiredeffect, for example, the prevention of brittleness, is too small if theamount of additive is insufficient and that the heat stability at thetemperatures encountered at high cutting speeds is reduced as the amountof additive is increased.

SUMMARY OF THE INVENTION

We have discovered a cutting tip having clearly improved performance. Inparticular, the cutting tip of the present invention exhibits a longercutting life and a greater output per cutting tip and per unit time. Theneed for such cutting tips arises particularly in the processing ofsteel parts and especially those which are to be processed fromcase-hardening steel and quenched and tempered steel, such as, shafts,axles, etc.

More particularly, the cutting tip of the present invention is composedof aluminum oxide with additions of zirconium oxide which is sintered athigh temperatures, wherein:

(a) the material composition is

70-90 weight percent of aluminum oxide,

10-30 weight percent of zirconium oxide,

0.1-0.5 weight percent of magnesium oxide with the other oxideimpurities amounting to less than 0.6% by weight, the proportions ofcomponents adding up to 100;

(b) the porosity is less than 2%;

(c) the average particle size is less than 1.7 μm; and

(d) the fracture toughness K_(Ic) at room temperature is at least 190N/mm³ /² and at least 140 N/mm³ /² at 1000° C., measured on prismaticrods cut 2.5 mm wide, 3.5 mm high and 12 mm long from the cutting tipsand provided with a saw cut 120 μm wide, 0.8±0.1 mm deep and having anotch radius of between 50 and 60 μm.

In accordance with another aspect of the present invention, the cuttingtips of the present invention are prepared by intimately mixing theabove-mentioned components in the amounts indicated with the addition ofmolding auxiliaries and, molding and sintering the mixture in thedesired geometric shape at a temperature in the range from about 1500 to1600° C. for a period of at least about 3 hours.

BRIEF DESCRIPTION OF THE DRAWING

The drawing is a graph of the variation of fracture toughness withtemperature for certain ceramic materials.

DESCRIPTION OF THE PREFERRED EMBODIMENT

German Offenlegungsschrift No. 25 49 652 discloses the use of sinteredobjects with a matrix of widely differing materials, including aluminumoxide and intercalated particles of unstabilized zirconium oxide for gasturbine components which require a high resistance to temperaturechanges. It was completely surprising, however, that, by the interactionof all the above-mentioned characteristics, cutting tip properties areachieved which make it possible to attain performances significantlyhigher than those attained by any previously known cutting tips andparticularly at high cutting speeds, i.e., 500 m/min and higher. Thiswas particularly surprising because the stresses present in cutting tipsare of a type completely different from those in gas turbines, forexample, and because cutting tips are subject to high wear, edgestresses and interactions with the materials being machined, whichgenerally are negatively affected by appreciable additions ofductilizing materials. It is difficult to establish which of thesecharacteristics is of greater importance.

It is, however, of considerable significance that the individualcomponents are present in a very pure form and that the magnesium oxidecontent does not exceed a particular value. The small amount of 0.1 to0.5 weight percent of magnesium oxide is sufficient for preventing theunrestrained growth of grains. The zirconium oxide, which is added tothe aluminum oxide, is present in the starting material in itsmonoclinic form. On the other hand, the small amounts of 0.1 to 0.5weight percent of MgO are not sufficient to stabilize the ZrO₂ duringsintering in its cubic modification.

In the sintering process, the unstabilized zirconium oxide is convertedinto the tetragonal phase. Being reversible, this phase is convertedback to the monoclinic form on cooling. It is probably of considerableimportance that the conversion to the monoclinic phase is restrained bythe relatively small particle size of less than 1 μm which, preferably,is even less than 0.5 μm. As a consequence, latent tensions areincorporated in the crystal structure which are suitable for absorbing alarge portion of the forces which occur in the metal-removingprocessing.

It is important that the cutting tip have a porosity of less than 2%,preferably of less than 1%. This increases the cutting performance andespecially the edge strength of the inventive cutting tip. Byappropriate procedures, it can be ensured that these pores are as smallas possible, preferably having a median value of less than 2 μm.

Moreover, very important effects result from the fracture toughnessK_(Ic) being at least 190 N/mm³ /² at room temperature and especiallythat at 1000° C., that is, at temperatures which are frequentlyencountered in machining operations at high cutting speeds, the fracturetoughness still is at least 140 N/mm² /². Preferably, the fracturetoughness is even greater than 200 N/mm³ /² at room temperature and atleast 150 N/mm³ /² at 1000° C.

The required small porosity of the inventive cutting tip goes hand inhand with a high density of the material. This density should be atleast 98% of the theoretical value. In the preferred case with azirconium oxide of 13 to 17 weight percent, the density of a cuttingtip, sintered in the usual manner, is preferably at least 4.1.

The high K_(Ic) value ensures that particles do not break off from thecrystalline system during the machining process or as a result of thehigh edge stresses which occur during the machining process.Consequently, the service life of the tool is increased and materialscan be machined at high speeds, which preferably could not beeconomically used with conventional cutting ceramics.

It was altogether surprising that a longer tool life can be achievedwith relatively high proportions of zirconium oxide, which would havebeen expected to reduce the temperature resistance, if all of the aboveproperty requirements are combined in the manner specified. By so doing,a grain structure is attained which has an optimum combination ofproperties relative to the previously known cutting tips.

At the same time, the dense and tight grain system is of particularimportance. With this property, even if the tool is overstressed, thebreaking off of complete corners from the cutting tip which would makeit unusable can be avoided.

The process for the preparation of the inventive cutting tip isimportant in order to achieve this special combination of properties,which was previously considered impossible to attain and which isresponsible for the significantly higher specific stress-carryingability of the cutting edges.

A preferred process comprises intimately mixing:

70 to 90 weight percent of aluminum oxide with an Al₂ O₃ of at least 99%and an average particle size of a median value of less than 1.0 μm;

10 to 30 weight percent of zirconium oxide with a ZrO₂ content of atleast 99% and an average particle size of a median value of less than1.0 μm; and

0.1 to 0.5 weight percent of a pure magnesium oxide, the oxideimpurities of this starting mixture of powders amounting to less than0.6 weight percent.

If desired, molding auxiliaries may be added and cutting tips of thedesired geometric shape are molded from the mixture and sintered attemperatures of 1500° to 1600° C. for a period of at least 3 hours.Preferably, the molded article is sintered at a sintering temperaturebetween 1520° C. and 1560° C. for a period of 6 to 10 hours.

The general relationship between the sintering time and sinteringtemperature is that longer sintering times are selected at lowersintering temperatures and shorter sintering times are used at highersintering temperatures. Below a sintering temperature of 1500° C.,however, the molded article generally no longer has the necessary highdensity and low porosity. Above 1600° C., on the other hand, the growthof the grain increases markedly, the average grain size in the finishedsintered product increases to more than 1.7 μm, the crystallinestructure becomes less homogeneous and consequently, the properties ofthe cutting tip deteriorate appreciably, especially at the high cuttingspeeds aimed for.

The necessity for using very pure starting materials has already beenstated. Particularly, advantageous results, especially in regard to theservice life of the cutting tip, are obtained when the purity of thealuminum oxide used as well as that of the zirconium oxide is evenhigher and the alumina content and the ZrO₂ content exceed even thevalue of 99.9%. It should be pointed out that the ZrO₂ content includesany hafnium oxide which may be present. It is well known that theproperties of these two elements and of their compounds are so similarthat they occur together in nature and can be separated from one anotheronly with difficulty. The hafnium oxide content, which may be as high as2%, is therefore usually not removed. References to 99 weight percentand 99.9 weight percent of ZrO₂ therefore relate to ZrO₂ +HfO₂.

At the same time, it is essential that the other oxide impurities, suchas, SiO₂, calcium oxide and the like constitute less than 0.6 weightpercent of the starting powder mixture and are, preferably, even anorder of magnitude below this value. In this respect, the object of thepresent application differs from previously known cutting tips based onaluminum oxide and zirconium oxide. In order to ensure this low value ofother oxide impurities, the magnesium oxide, which is added as a graingrowth inhibitor, must, of course, also be very pure. Advantageously, analuminum oxide powder is used to which the required amount of puremagnesium oxide has already been added during the manufacture.

The initial size of the pulverulent starting materials also has a verysignificant effect. In order to arrive at the very fine and uniformgrain structure, the median value of the aluminum oxide and of thezirconium oxide should lie below 1.0 μm and, in the case of thezirconium oxide, preferably even below 0.5 μm. The medium value isdefined as follows. The particle size distribution is represented in theusual manner by a cumulative distribution by plotting the size parameteragainst the percentage frequency up to 100%. Corresponding frequencydistribution curves are characterized simply by stating the medianvalue. The median value is defined as the intersection of the cumulativecurve with the 50% line of the frequency function and therefore denotesthe average size (particle size or pore size), 50% of the particles orpores being larger and 50% being smaller than this medium value.

The starting powder mixture is advisably molded at a pressure of atleast 6000 N/cm² and preferably at one of 12,000 to 16,000 N/cm². Thereis a certain regularity here also, in that higher sintering temperaturesare selected at lower pressures and lower sintering temperatures athigher pressures.

Widely differing materials are suitable as molding auxiliaries.Particularly good results have been achieved with polyvinyl alcoholswhich are advantageously used in an amount of weight percent. Themolding moisture of the starting powder is about 4% by weight.

The quality of the cutting tip, particularly its high density and lowporosity, is also affected to a considerable degree by the heating andcooling rates, that is, the time during which the molded green tip isheated to the stated sintering temperature and the time during which itis cooled once again to room temperature. Preferably, these heating andcooling rates are about 200° l C./hour.

It is best if the inventive cutting tips are sintered in an oxidizingatmosphere. This represents a significant advantage over previouslyknown cutting tips which contain carbide or nitride additives andtherefore can be sintered only in a reducing or an inert atmosphere orin a vacuum. This, of course, requires a much greater expenditure forequipment and makes these cutting tips very expensive. Obviously, theinventive cutting tips can also be sintered by different procedures, forexample, by the hot pressing process. In this case, the sintering timesare significantly shorter and the temperatures in the section higher byabout 100° C.

As has already been stated several times, the high K_(Ic) value andespecially, the value at 1000° C. which is the temperature occurring athigh cutting speeds, is a very significant characteristic of theinventive cutting tips. The drawing shows a comparison of the K_(Ic)values of a ZrO₂ and TiC-containing Al₂ O₃ sintered ceramic, similar tothat described in German Offenlegungsschrift 27 41 295 (curve 1) withthose of the inventive product (curve 2). It can be seen clearly thatthe K_(Ic) values are largely the same at room temperature and that theK_(Ic) value of the prior art material decreases much more rapidly withincreasing temperature than that of the invention.

The determination of the K_(Ic) value (critical stress intensity factor)which is a measure of fracture toughness is described in the following.

Sample Dimensions:

Prismatic rods, with a width B of 2.5 mm, a height W of 3.5 mm and alength L of 12 mm, are cut to size from the cutting tip. Subsequently, acrack-like saw cut with a width d=120 μm, and a depth a=0.8±0.1 mm ismade over the width B at L/2 with a diamond-studded copper disc. Thenotch radius is between 50 and 60 μm.

Procedure:

The notched sample is stressed until it fractures in a three-pointbending test. For this purpose, it is placed on edge on supports at adistance S=11 mm apart and is acted upon by the test load on the sideopposite to the saw cut. The load deflection rate at the site of highestbending moment is 0.25 mm/min.

The fracture resistance K_(Ic) is determined from the equation

    K.sub.Ic =σ.sub.bB ·√a·y

in which

    σ.sub.bB =(3SF.sub.B /2BW.sup.2)

σ is the bending strength relative to the cross-section (W.B) y atabulated constant, which depends only on the a/W ratio, and F_(B) isthe breaking load.

The dimensions of the samples and the procedure are based on the ASTMMethod E 399-72 of the Standard Test Methods for determining the tensilestrength of metallic materials. With respect to the adjustment to thepresupposed level state of extension and to the formation of plasticzones, ceramic materials behave uncritically. The simulation of sharpcracks by fine saw cuts represents a normal simplification for measuringthe K_(Ic) value of ceramic materials (see T. R. Wishaw et al, Eng.Fract. Mech. 1, 1968, 191, R.F. Pabst, dissertation, Stuttgart 1972, R.L. Bertolotti, 2. Amer. Ceram. Soc. 56, 1973, 107).

The preparation of a cutting tip in accordance with the presentinvention is described in the following example.

Alumina (42 kg), containing 0.2 weight percent of magnesium oxide, isdispersed with 7.5 kg of unstabilized zirconium oxide in distilled waterand ground in a vibrating ball mill for 30 minutes in order to achievehomogeneous mixing. After the grinding, 1% of polyvinyl alcohol is addedand the suspension is stirred intensively for 10 minutes at 500 rpm andsieved through a 40 μm sieve. The sieved suspension is dried in a spraydrier and granulated. The granulated and resultingly flowable powder ismolded in powder presses with a pressure of 12,000 N/cm² into cuttingtips of the desired geometric shape with a green density of 2.52 g/cm².After this molding process, the molded articles are sintered at 1550° C.for a period of 8 hours. The heating and cooling rates were 200° C./hr.After the sintering, the cutting tips are ground by machining withdiamond grinding wheels to their final cutting-tip geometry, inaccordance with the Standard Regulations SNG, and mounted.

A cutting tip, prepared according to this example, has the followingproperties:

a density of 4.17 g/cm³ ;

a porosity of 0.5%;

an average grain size with a median value of 1.45 μm;

a Vickers hardness of 19700 N/mm² ;

a bending strength of 580 N/mm² ;

a fracture toughness of 201 N/mm³ /² at room temperature; and

a fraction toughness of 150 N/mm³ /² at 1000° C.

All grain size data in this description was determined by theSyner-Graff method.

Such a cutting tip withstands a significantly higher specific load onthe cutting corners. The load carrying capacity is tested by theso-called feed test, which is carried out as follows.

At a given depth of cut (in this case 3 mm) and at a particular settingangle (in this case 85°) and for a specific cutting tip geometry (inthis case Type SNGN 120816 T -0.2×20°), the feed rate is increased untilthe cutting tip is just able to survive without breaking a cutting timeof 10 minutes at a cutting speed of 500 m/min.

Under the aforementioned test conditions, a feed rate of 0.70mm/revolution was obtained for the cutting tip of the example.

A feed rate, which is so much higher than that of previously knowncutting tips whose construction is based on Al₂ O₃ alone or on ZrO₂ andTiC-containing cutting ceramics in accordance with GermanOffenlegungsschrift No. 27 41 295, in practical terms means asignificant increase in the number of parts which can be machined withone cutting tip. For instance, using a rear-axle made from 41 Cr4V90 asan example, the number of parts which can be machined with one cuttingtip is increased sixfold over the number of parts which can be machinedwith one cutting tip of alumina or of Al₂ O₃ -ZrO₃ -ZrO₂ -TiC ceramic,fabricated in accordance with German Offenlegungsschrift No. 2741295.

What is claimed is:
 1. A cutting tip for shaping and metal removalduring the manufacture of metal parts comprising the followingcomposition:70-90 weight percent of aluminum oxide; 10-30 weight percentof zirconium oxide; 0.1-0.5 weight percent of magnesium oxide;whereinthe amount of other oxide impurities is less than 0.6 weight percent andthe proportions of components add up to 100, said composition beingsintered at high temperatures and having a porosity of less than 2%; anaverage particle size of less than 1.7 μm; and a fracture toughnessK_(Ic) at room temperature of at least 190 N/mm³ /² and of at least 140N/mm³ /² at 1000° C., measured on prismatic rods 2.5 mm wide, 3.5 mmhigh and 12 mm long, cut from the cutting tips and provided with a sawcut 120 μm wide, 0.8±0.1 mm deep and having a notch radius of between 50and 60 μm.
 2. The cutting tip of claim 1 wherein the average particlesize is between 1.4 and 1.6 μm and 90% of all particles lie within thissize range.
 3. The cutting tip of claim 1 or 2 wherein the fracturetoughness K_(Ic) is at least 200 N/mm³ /² at room temperature and atleast 150 N/mm³ /² at 1000° C.
 4. The cutting tip of claim 1 or 2wherein the zirconium oxide content is from about 13 to 17 weightpercent and the density is at least 4.1 g/cm³.
 5. The cutting tip ofclaim 1 or 2 wherein the porosity is less than about 1%.
 6. The cuttingtip of claims 1 or 2 wherein the median pore size is less than 2 μm. 7.A process for the production of a cutting tip comprising intimatelymixing:70-90 weight percent of aluminum oxide with an Al₂ O₃ content ofat least 99% and an average particle size of a median value of less than1.0 μm; 10-30 weight percent of zirconium oxide with a ZrO₂ content ofat least 99% and an average particle size of a median value of less than1.0 μm; and 0.1-0.5 weight percent of pure magnesium oxide, the oxideimpurities of this starting powder mixture amounting to less than 0.6weight percent;molding cutting tips of the desired geometrical shapefrom the mixture and sintering the molded mixture at a temperature of1500° to 1600° C. for a period of at least 3 hours and then cooling thesintered product.
 8. The process of claim 7 wherein the molded mixtureis sintered at a temperature from 1520° to 1560° C. for a period of from6 to 10 hours.
 9. The process of claim 7 or 8 wherein the rate ofheating to the sintering temperature and the rate of cooling to roomtemperature after sintering is about 200° C. per hour.
 10. The processof claim 7 or 8 wherein the sintering is carried out in an oxidizingatmosphere.
 11. The process of claim 7 or 8 wherein the median value ofthe particle size of the zirconium oxide is less than about 0.5 μm. 12.The process of claims 7 or 8 wherein the mixture is molded at a pressureof at least 6000 N/cm².
 13. The process of claim 7 or 8 wherein themixture is molded at a pressure of at least 12,000 to 16,000 N/cm². 14.The process of claims 7 or 8 wherein a molding auxiliary is mixed intothe mixture prior to molding.
 15. The process of claim 14 wherein themolding auxiliary is a polyvinyl alcohol.
 16. In a method formanufacturing an article from a metal selected from the group consistingof case-hardened steel, quenched steel and tempered steel, wherein thesteel is processed with a cutting tip, at cutting speeds greater than500 m/min, the improvement which comprises said cutting tip beingcomposed of the following composition70-90 weight percent of aluminumoxide, 10-30 weight percent of zirconium oxide, 0.1-0.5 weight percentof magnesium oxide,wherein the amount of other oxide impurities is lessthan 0.6 weight percent and the proportions of components add up to 100,said composition being sintered at high temperatures and having aporosity of less than 2%, an average particle size of less than 1.7 μm,and a fracture toughness K_(Ic) at room temperature of at least 190N/mm³ /² and of least 140 N/mm³ /² at 1000° C., measured on prismaticrods 2.5 mm wide, 3.5 mm high and 12 mm long, cut from the cutting tipsand provided with a saw cut 120 μm wide, 0.8±0.1 mm deep and having anotch radius of between 50 and 60 μm.
 17. A composition comprising70 to90 weight percent of aluminum oxide; 10 to 30 weight percent ofzirconium oxide; 0.1 to 0.5 weight percent of magnesium oxide;whereinthe amount of other oxide impurities is less than 0.6 weight percent andthe proportions of components add up to 100, said composition beingsintered at high temperatures and having a porosity of less than 2%, anaverage particle size of less than 1.7 μm, and a fracture toughnessK_(Ic) at room temperature of at least 190 N/mm³ /² and of at least 140N/mm³ /² at 1000° C., measured on prismatic rods of said sinteredcomposition 2.5 mm wide, 3.5 mm high and 12 mm long and provided with asaw cut 120 μm wide, 0.8±0.1 mm deep and having a notch radius ofbetween 50 and 60 μm.